Every year around a billion birds—an estimated 5 percent of the country's bird population—die each year by crashing into windows they cannot see, according to Professor Daniel Klem, Jr., an ornithologist at Muhlenberg College in Pennsylvania who has spent over 30 years studying bird collisions. Migratory birds, including songbirds whose populations are already on the decline, crash into glass in especially large numbers. See Bina Venkataraman, Fatal Reflections: As ‘green’ architecture advances, glass buildings pose hazard to birds, Globe Newspaper Company, Dec. 15, 2008. While environmentalists often express concern about the effects of oil spills, wind turbines, and cell phone towers, Professor Klem says that only habitat destruction kills more birds than window collisions. See Joann Loviglio, Plate glass blamed for a billion bird deaths a year, Associated Press, Feb. 2, 2004.
Two properties of glass that make it so appealing—the way it reflects light and allows humans to see through it—are the very reasons that birds crash into it. They see mirror-images of trees and sky they want to fly toward, or they may see food, water, or habitat inside or beyond the windows. See Bina Venkataraman, Fatal Reflections, supra.
The danger of bird collisions is increasing as cities erect buildings that emphasize natural light, and as improvements in the energy efficiency of glass increase its use. If not patterned, tinted, or used in small panes, low-e glass has a mirror-like quality. Green roofs reflected on surrounding buildings can lure birds into walls.
Current solutions for avoiding bird strikes on both residential and commercial buildings are not effective. Home owners are told to relocated bird feeders; cover windows on opposite walls with paper or cardboard; attach stickers, netting, or dead tree branches to outside glass; install awnings; or configure windows to tilt downwards. The remedies for saving the birds that enthusiasts want to watch generally have the adverse effect of preventing the enthusiasts from seeing the birds.
Although the cities of Toronto and Chicago and the New York City Audubon Society have published bird safe building guidelines, commercial builders have not been able to help birds without either interfering with the view from inside or substantially increasing costs. Some buildings have utilized bird-safe design recommendations that encourage features such as fritted or patterned glass and ceramic rods placed over the windows. The Illinois Institute of Technology's student center in Chicago has a dot matrix pattern in its glass that makes it less transparent and reflective to birds but is hard for people to see through unless they are standing at a distance from the windows. The German company Arnold Glas has developed a striped glass that costs about 30 percent more than other high performing glass but still obstructs the view.
It is desirable to make windows identifiable for flying birds. Birds are able to see light in the ultraviolet (UV) range, specifically in a wavelength band from about 300 to 400 nm that is invisible to people. According to Prof. Klem, research indicates that covering windows on the outside surface with visual cues whose elements are apart two inches horizontally to four inches vertically eliminates birds strikes altogether. Patterns can be any object, including circles, lines, stripes, hawk silhouettes, or UV-reflecting leaves. It is the spacing that is most important. See Daniel Klem, Jr., Windows: an unintended fatal hazard for birds, available at www.savingbirds.org. It is also desirable that the cues are not visible to humans.